1. Field of the Invention
The present invention relates to a semiconductor device and, more specifically, to a semiconductor device including a high-breakdown voltage MOS transistor.
2. Description of the Background Art
Semiconductor devices having a high-breakdown voltage MOS (Metal Oxide Semiconductor) transistor applied to a logic circuit or an analog circuit, have been known. As an example of such a semiconductor device, a semiconductor device disclosed in Japanese Patent Laying-Open No. 2001-94103 will be described.
In the semiconductor device described in this laid-open application, in one of prescribed regions on a semiconductor substrate, an n-channel type high-breakdown voltage MOS transistor is formed.
First, in a P-type semiconductor device, a P-type well is formed. The P-type well is a well diffusion layer for the high-breakdown voltage MOS transistor. On the P-type well, a gate electrode is formed with a gate oxide film interposed.
Between the gate electrode and the gate diffusion layer and between the gate electrode and the source diffusion layer, an LOCOS (Local Oxidation of Silicon) oxide film is formed. The LOCOS oxide film separates the gate electrode from the drain diffusion layer at the surface, and separates the gate electrode from the source diffusion layer at the surface.
Immediately below the LOCOS oxide films at opposing ends of the gate electrode, a drain side offset region and a source side offset region are formed, respectively. Below the drain diffusion layer, the drain side well offset region is formed. Below the source diffusion layer, the source side well offset region is formed.
The gate, drain and source are electrically isolated from a channel stopper region serving as a diffusion layer to take a potential of the P-type well diffusion layer, by an N-type isolating diffusion layer, a P-type isolating diffusion layer and the LOCOS oxide film. The channel stopper is formed to surround the high-breakdown voltage MOS transistor.
The conventional semiconductor device including a high-breakdown voltage MOS transistor is formed in the above described manner.
The conventional semiconductor device described above, however, has the following problems. When the high-breakdown voltage MOS transistor is to be applied to a logic circuit such as an NAND circuit or an NOR circuit, or when it is to be applied to an analog circuit, it is necessary to connect the high-breakdown voltage MOS transistors in series.
When the above described high-breakdown voltage MOS transistors are to be connected in series, the source/drain of a high-breakdown voltage MOS transistor formed in one channel stopper must be connected, for example, by an aluminum interconnection, to the source/drain of a high-breakdown voltage MOS transistor formed in a different channel stopper.
At this time, the high-breakdown voltage MOS transistors are connected in series by repeatedly arranging the region (pattern) in which the high-breakdown voltage MOS transistor including the channel stopper is formed.
As the pattern is arranged repeatedly, the area occupied by the pattern on the semiconductor substrate becomes large, and the pattern layout area of the semiconductor device as a whole becomes undesirably large.
In a circuit in which a resistance element is connected to a high-breakdown voltage MOS transistor, the resistance element that is connected to the high-breakdown voltage MOS transistor must also have high breakdown voltage.
In order to ensure a high breakdown voltage, as a resistance element, a resistance element formed of polysilicon film is sometimes formed on the LOCOS oxide film. The resistance element formed in this manner is connected to the source/drain of the high-breakdown voltage MOS transistor through an aluminum interconnection.
When the resistance element is connected in series to the high-breakdown voltage MOS transistor, again, the region to form the resistance element of polysilicon film on the LOCOS film must be ensured. Accordingly, the pattern layout area of the semiconductor device as a whole becomes undesirably large.